The present invention relates to novel amine-terminated hyperbranched quinoxaline-amide polymers as room-temperature initiators for bismaleimide polymerization.
Historically, fabrication of high performance, organic matrix composite (OMC) structures for aircraft and space systems applications on a low volume basis is very costly. This is because nonrecurring costs such as tooling and capital equipment are the major cost drivers for low volume production. Thus, there is a pressing need to reduce the costs of OMC structures, for both prototype and low-volume programs. Since autoclaves and hardened tooling, traditionally required for the fabrication of large, composite-based spacecraft components, have the lion""s share in the fabrication cost, the affordability issue can be logically addressed by developing non-autoclave resins and processing. Toward this end, there are currently two approaches, namely, low-temperature curing and electron-beam (E-beam) curing. In the former, the objective is to drastically lower the curing temperature and pressure for composites so that tooling can be fabricated easily from relatively inexpensive materials, such as wood, fiberglass, or foam. However, for these processing conditions, the material systems (prepreg, liquid resin and adhesive) must possess characteristics that are conducive to low temperature/low pressure processing (e.g., 150xc2x0 F. and 14 psi), and, after post-cure in free-standing fashion, should provide structural performance equivalent to current aerospace standards (e.g. epoxy-3501). This approach is hampered by the lack of suitable material systems that can be cured at temperatures below 65xc2x0 C. to form structures with high temperature properties. An alternative approach to low-temperature thermal cure is E-beam curing. Although this method of curing is rapid (seconds to minutes as opposed to hours for thermal curing), a high-energy, electron-beam source ( less than 250 KeV to  greater than 1 MeV) is required, necessitating some measures of personnel protection.
Bismaleimide (BMI) resins are attractive for composite applications because such resins can be processed and fabricated similar to epoxies and their use temperatures are much higher. However, the curing temperatures of BMI are generally in excess of 200xc2x0 C. Therefore, it is desirable to lower the polymerization temperatures to below 65xc2x0 C. and at the same time preserve the high temperature properties from conventional thermal cure of BMI resins.
Dendritic macromolecules such as dendrimers and hyperbranched polymers are a new class of highly branched polymers that have distinctly different properties from their linear analogs Both dendrimers and hyperbranched polymers have much lower solution and melt viscosities than their linear analogs of similar molecular weights. They also have a large number of chain-ends whose collective influence dictates their overall physical and/or chemical behaviors. These features are attractive in terms of processability and offer flexibility in engineering-required properties for specific applications.
Hyperbranched polymers have an important and practical advantage over dendrimers at the xe2x80x9craw materialxe2x80x9d level. Although dendrimers have precisely controlled structures (designated as generations), their preparations generally involve tedious, multi-step sequences that are impractical and costly in scale-up production. Synthesis of a hyperbranched polymer, on the other hand, is a one-pot process. Large quantities of hyperbranched polymers can be easily produced from ABx (x xe2x89xa72) monomers.
Accordingly, it is an object of the present invention to provide novel quinoxaline-containing monomers AB2 monomers for hyperbranched aromatic polyamides.
Other objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
In accordance with the present invention there are provided AB2 monomers for hyperbranched aromatic polyamides of the formula 
wherein Q is 
The AB2 monomers of this invention can be synthesized as described in the Examples which follow.
These monomers can be polymerized to provide polymers having the following repeating units: 
Aromatic polyamides are prepared via two general routes: (i) polycondensation reaction via an aromatic diacid chloride and a diamine and (ii) direct polycondensation reaction of a dicarboxylic acid and a diamine. For route (i), because of the extreme moisture sensitivity of diacid chlorides and the highly exothermic nature of the reaction between an amine and a carboxylic acid chloride, the polymerization is usually conducted at temperatures at or below 0xc2x0 C. and under inert atmosphere. In addition, in order to consistently achieve high molecular weight for the resulting polyamides, the diacid chloride monomers must be freshly purified prior to polymerization. For route (ii), since the dicarboxylic acid monomers are cheaper, much less sensitive to moisture and relatively easy to purify via recrystallization, the polycondensation is more amenable to scale-up. However, because of little or no formation of amide from a carboxylic acid and an amine under ambient conditions, a phosphorus (V)-based promoter is used to activate the carboxylic group. The Yamazaki reaction embodies the most commonly used conditions that employ triphenyl phosphite (TPP) in N-methylpyrrolidinone (NMP) solution containing lithium chloride or calcium chlorides at 100xc2x0 C.
The polymers can be employed to initiate bismaleimide polymerization. Another application of these polymers is to increase the toughness for thermosets such as BMI and epoxies. By the term toughness is meant resistance to impact induced damage. Toughness in cured neat resin samples may be assessed by the critical stress intensity factor, K1C, among others. Toughness in fiber reinforced composites prepared by laying up and subsequently curing numerous plies of prepregs is best assessed by measuring the compression strength after an impact of suitable energy. Generally, an impact of 1500 in-lb/in is used, and compression after impact (CAI) values measured in accordance with Boeing test BSS 7260 on a quasiisotropic [+45/0/xe2x88x9245/90]4s layup. Alternatively, other measures of toughness such as laminate GIIC are used.
The following examples illustrate the invention: